There is a continuing trend throughout the world toward consumption of more fresh and minimally processed food. High quality fresh fruits and vegetables are now available year round, thanks to improved packaging, storage technologies and rapid global transportation. The abundance of year-round fresh produce is dependent on a vast infrastructure including specialized refrigerated storage facilities.
Maintaining the freshness of fruit, vegetables, and other horticultural products such as fresh cut flowers is very important to the postharvest industry and producers during various stages of transportation and storage. One of the ways to control the freshness of produce is by regulating its exposure to ethylene. Plants are very sensitive to ethylene concentration mainly because ethylene is one of their growth hormones. When produce has a limited exposure to ethylene, its natural aging process will be slowed. Yet, if ethylene concentrations reach a high enough level, produce will not only age faster, but begin to decay.
Ethylene production rate and the amount of ethylene present in the environment surrounding a single apple or pear (or in general for climacteric fruit) have been shown to affect the quality of these fruit during various stages of ripening. This is especially true post-harvest, where the rate of ripening, scalding, browning, and other issues could prevent high quality fruit from reaching the market.
A number of researchers are currently using various methods supported by a Gas Chromatography system (GC) to research the different aspects of interaction of ethylene and fruit quality at various pre- and post-harvest stages. While significant amount of data has been accumulated and a large of body of literature exists on varieties such as Bartlett pears and golden delicious apples, little information is available for some of the newer varieties such as Comice pears (and honey crisp apples). Research performed on Bartlett pears suggested that very low ethylene concentrations of less than 1 ppm have to be maintained to control fruit quality, which is difficult due to high ethylene production of fruit even at −1° C. storage temperatures. For such tight control, continuous monitoring of the ethylene levels in the storage facilities is required. There is currently no cost-effective real-time ethylene sensor in the market that can produce reliable measurements of ethylene at 0.1 ppm, the level required for control in storage areas.
In general, the ethylene-related problems result when coexistence of high ethylene-producing fresh fruit and vegetables (FF&V) are placed in the same storage area with highly ethylene sensitive FF&V (or cut flowers). For example, avocados and apples are known to produce extremely high ethylene levels even at less than 4° C. On the other hand, kiwifruit is not a high ethylene producer, but is extremely sensitive to the presence of ethylene and should not be stored where it might be exposed to significant amounts of ethylene. As little as 5 to 10 ppb (0.005 to 0.010 ppm) ethylene in a storage atmosphere can accelerate softening without impacting other ripening processes. This results in unripe fruit that are excessively soft. Carrots produce very small amounts of ethylene at (<0.1 μL kg−1 h−1 at 20° C.). However, exposure to exogenous ethylene (˜0.2-ppm) will induce development of isocoumarin and bitter flavor in carrots. While separating the various fruit and vegetables in cold storage may seem like a logical approach to cold storage, it is impractical to have a separate cold storage area for every cultivar of FF&V.
Ethylene monitoring is currently not a widely adopted process in many packing houses and cold storage facilities. Some ethylene sensors are limited in detection accuracy and those with significant accuracy are too large (suitcase size) and too expensive (several thousand dollars) for packing houses to afford and use. Localization of more rapidly ripening fruit that is the source of ethylene is challenged by the high cost and inconvenience in detection. Such localization could provide strategies to minimize ethylene production and to control spoilage and rapid ripening process.
These problems can be addressed if a cost effective, preferably compact ethylene detection method is made available to warehouses and/or to growers for monitoring ethylene in storage environments and in orchards to monitor the ripening process prior to harvesting.
The most basic existing technology for ethylene measurement is to take an air sample, then later test it at one's convenience for ethylene concentrations under laboratory conditions. For example, an air sample may be gathered in a sample bag and sent to the lab for testing. This technique gives one measurement of ethylene concentration whose accuracy is only limited in accuracy by the way that the sample is taken and tested. The main draw back to this technique is that the ethylene concentration is not known in real time—there is a delay associated with the sampling and testing. Due to the cumbersome nature of the process, this technique is not practical for continuous ethylene monitoring.
Another current technology for ethylene measurement is to use a sampling pump to draw air through a detector tube. A detector tube is a small tube that when air is pumped over it, the concentration of a particular gas is indicated. This is normally done by means of a color change shown on graduations along the side of the detection tube. The resolution of this technique is only as good as one can read the color change. This technique is also limited by the use of one time, disposable tubes. The detection tube can be exposed to air either by means of a hand pump (such as Sensidyne's AP-1S) or by a mechanical pump that draws air more slowly across the detection tube, to provide a reading averaged over a longer period (such as Sensidyne's GilAir5). Again, this technology is not suitable for continuously monitoring ethylene concentrations in environments such as de-greening rooms.
Personal air samplers are a very commonly used technology for measuring ethylene concentrations in de-greening rooms. These are often hand held or belt clip air samplers that give real time information on various gas concentrations. The detection is accomplished by using a metal oxide such as tin oxide to detect changes in surface resistance as a gas is adsorbed onto the surface. Although this technique can provide real time information, it has a limited resolution of 1 ppm or higher.
Another ethylene sensing technology is based on the chemiluminescence reaction of ethylene and ozone. Chemiluminescence of the ozone-ethylene reaction has been extensively studied and is well-documented in the literature. Most of these studies were triggered by the desire to accurately measure the ozone level in the atmosphere or for the process industry where monitoring and control of ozone is important. Surprisingly, the reverse has been less of interest to most of the researchers with the exception of one study and group at Geo-Centers, Inc. In summary, the reaction of ethylene with ozone produces a number of intermediate products, including the light emitting species OH+ and HCHO* at excited state. When these intermediate species decay, they release energy in the form of electromagnetic (EM) radiation or photons, with energy of hν. Detailed spectra of the emissive power from these decaying molecules reveal EM radiation energy at several different wavelengths, ranging between UV to IR, including visible radiation.
U.S. Pat. No. 6,105,416 describes an ethylene detector based on chemiluminescence. This detector requires that the ethylene sample and ozone be pumped concurrently into a pressurized test chamber that also has ozone concurrently being pumped in under pressure. Pressurized ozone is used because a higher reagent concentration of ozone increases the likelihood that the ozone will react with ethylene, thus increasing the system's efficiency and signal to noise ratio. The ozone used is created internally by means of a separate ozone generator that is fed either compressed air, or compressed oxygen. These ethylene detection systems, operating by means of discrete test chambers, ozone generators, and various valving for the pressurized gasses tend to be large, cumbersome and very expensive. Furthermore, generation of ozone in a high-pressure oxygen or air environment poses the risk of explosion and can be deemed hazardous. Because of cost limitations, only one system within the entire de-greening building (with individual sampling lines routed to each degreening room) can be used, making it unsuitable and expensive for localization of the ripening process.
What is needed is an inexpensive, light-weight, portable sensor that is capable of accurately detecting the presence of a target organic molecule in a sample gas such as air at concentrations of less than 1 ppm.